1. Field of the Invention
The present invention relates to a directional coupler, and more specifically, to a directional coupler which is capable of reducing the operating frequency thereof, improving the degree of electromagnetic coupling between a main line and a sub-line, and reducing the height thereof, and which facilitates impedance design of respective terminals.
2. Description of the Related Art
For example, a known directional coupler is disclosed in Japanese Unexamined Patent Application Publication No. 8-237012 as including a laminate block in which a plurality of dielectric layers including coil conductors or ground conductors disposed thereon are laminated. Two coil conductors are provided inside the laminate block, with one of the coil conductors defining a main line and the other coil conductor defining a sub-line. Further, the main line and the sub-line are electromagnetically coupled to each other. Further, the ground conductors sandwich the coil conductors in a lamination direction.
In the directional coupler having the above-described configuration, upon input of a signal to one end of the main line, a signal having power proportional to the power of the input signal is output from one end of the sub-line.
There is a case in which it is desirable to reduce the operating frequency of such a directional coupler. In such a case, a method of increasing the line lengths of the main line and the sub-line is conceivable. However, according to the method, it is necessary to increase the area of the dielectric layers on which the main line and the sub-line are disposed. Thus, a problem arises in that the size of the directional coupler must be increased.
In view of the above, another known directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 2003-69317 uses a method of dividing both of the main line and the sub-line in different layers inside the laminate block, to thereby increase the line lengths of the coil conductors.
FIG. 6 illustrates a directional coupler 400 disclosed in Japanese Unexamined Patent Application Publication No. 2003-69317. FIG. 6 is an exploded perspective view of the directional coupler 400.
The directional coupler 400 includes a laminate block 101 including a plurality of laminated dielectric layers 101a to 101g. 
Further, a coil conductor 102a provided on a surface of the dielectric layer 101c, a via conductor 102b provided through the dielectric layer 101d, a via conductor 102c provided through the dielectric layer 101e, a via conductor 102d provided through the dielectric layer 101f, and a coil conductor 102e provided on a surface of the dielectric layer 101f are sequentially connected to define a main line. In the laminate block 101, the main line is divided into a first main line defined by the coil conductor 102a and a second main line defined by the coil conductor 102e. 
Similarly, a coil conductor 103a provided on a surface of the dielectric layer 101b, a via conductor 103b provided through the dielectric layer 101c, a via conductor 103c provided through the dielectric layer 101d, a via conductor 103d provided through the dielectric layer 101e, and a coil conductor 103e provided on a surface of the dielectric layer 101e are sequentially connected to define a sub-line. In the laminate block 101, the sub-line is divided into a first sub-line defined by the coil conductor 103a and a second sub-line defined by the coil conductor 103e. 
Further, the first main line (coil conductor) 102a and the first sub-line (coil conductor) 103a are electromagnetically coupled to define a first coupling portion 104, and the second main line (coil conductor) 102e and the second sub-line (coil conductor) 103e are electromagnetically coupled to define a second coupling portion 105.
Further, ground conductors 106a, 106b, and 106c are provided on a surface of the dielectric layer 101a, a surface of the dielectric layer 101d, and a surface of the dielectric layer 101g, respectively. Each of the ground conductors 106a, 106b, and 106c functions as a shield. Particularly, the ground conductor 106b is intended to prevent the occurrence of unnecessary signal leakage between the first coupling portion 104 and the second coupling portion 105. A central portion of the ground conductor 106b is provided with an opening to allow the via conductor 102b and the via conductor 103c to pass therethrough.
In the existing directional coupler 400 having the above-described structure, the main line and the sub-line are both divided in different layers inside the laminate block 100, to thereby allow an increase in line length of the coil conductors without a reduction in dimension of the elements in a planar direction.
However, in the above-described known directional coupler 400, the ground conductor 106b is provided on substantially the entire surface of the dielectric layer 101d to prevent coupling between the first coupling portion 104 and the second coupling portion 105. As a result, the following problem arises.
That is, the ground conductor 106b is provided on substantially the entire surface of the dielectric layer 101d, and the first main line 102a and the second sub-line 103e both face the ground conductor 106b. Therefore, there arises a problem in that it is difficult to optimize impedance characteristics of an output end derived from the first main line 102a and impedance characteristics of a coupling end derived from the second sub-line 103e. 
For example, to reduce the impedance value of the output end derived from the first main line 102a and the impedance value of the coupling end derived from the second sub-line 103e, it is necessary to increase the thickness of the dielectric layer 101d and thereby increase the distance between the ground conductor 106b and the first main line 102a, and to increase the thickness of the dielectric layer 101e and thereby increase the distance between the ground conductor 106b and the second sub-line 103e. In this case, there arises a problem in that the height dimension of the laminate block 101 is increased.